Shunt reactors causing CT failures?
Shunt reactors causing CT failures?
(OP)
We have a 69kV delta / 24.9kV gnd wye substation with a feeder that goes to an island. In the fall of 2008, we installed a 3 MVAR shunt reactor bank to counteract the ~3 MVAR of capacitance from the ~24 miles of submarine cable that feeds the island. This seemed to fix all issues we had.
This last fall, (Oct '09) we had an issue where a 50:5, RF 3.0 CT in the substation failed and fried our entire metering circuit. We replaced everything, but had no idea of the cause. A few months later a VT failed, and we chalked that up to damage sustained from the CT failure. A few days ago, we again had a total CT failure on B phase that again fried the entire metering circuit. We can find no data to suggest what might be the cause of these problems. There was no fault on the feeder, and from everything we can find, the sub was operating in normal conditions, no excessive loading. We have just replaced all of the metering transformers as well as the metering circuit.
It has been brought up that perhaps the reactors might be causing an abnormality. I can't think of any reason that they would since they are in continuous operation and the only time they are de-energized is then there is a recloser operation, in which case the capacitive cable is also dropped. But again, there was no reclosing event when the CT failed.
Any help, insight, or general conjecture on what might be happening would be greatly appreciated!
Thanks,
Kevin
This last fall, (Oct '09) we had an issue where a 50:5, RF 3.0 CT in the substation failed and fried our entire metering circuit. We replaced everything, but had no idea of the cause. A few months later a VT failed, and we chalked that up to damage sustained from the CT failure. A few days ago, we again had a total CT failure on B phase that again fried the entire metering circuit. We can find no data to suggest what might be the cause of these problems. There was no fault on the feeder, and from everything we can find, the sub was operating in normal conditions, no excessive loading. We have just replaced all of the metering transformers as well as the metering circuit.
It has been brought up that perhaps the reactors might be causing an abnormality. I can't think of any reason that they would since they are in continuous operation and the only time they are de-energized is then there is a recloser operation, in which case the capacitive cable is also dropped. But again, there was no reclosing event when the CT failed.
Any help, insight, or general conjecture on what might be happening would be greatly appreciated!
Thanks,
Kevin






RE: Shunt reactors causing CT failures?
David Castor
www.cvoes.com
RE: Shunt reactors causing CT failures?
Also, the recloser is in your substation, correct?
Do you have any switched capacitors in the substation or at the island-end?
Do you have any voltage monitoring in the substation, like Dranetz?
Some details on the cable would also be of interest like size, insulation type and mil thickness (260 mils is standard for 25 kV).
RE: Shunt reactors causing CT failures?
As the metering circuits were fried it suggests that the output of the CT went high. If it was a wound CT then I would say that there may have been an HV interturn fault but I'm not familiar with the construction of RF CTs. Have you spoken to the CT manufacturer or sent a failed CT back to them? It may a type fault.
RE: Shunt reactors causing CT failures?
CT failure mode is unknown. We cannot pinpoint any one mode of failure because everything was ruined. I would agree with your initial thoughts, as I cannot see any relation to the reactors and the failures. We are leaning toward poor grounding as the culprit, but the reactors were mentioned so I am trying to get any info I can.
magoo2-
The reactor is on the mainland side of the cable.
The recloser is in the substation (it is a 3 phase).
We have no switched capacitors to my knowledge on the circuit. We maintain the sub, but not the line, so we have limited information on anything outside the fence.
We just installed a PQ meter yesterday. We did this after the first CT failed but it never showed anything useful.
The only info I have on the cable is 4/0 copper. Again, not ours.
LSpark-
No primary fault that I was informed of. We are sending this CT in for an analysis. The RF is just rating factor, not type. The type was a JKW-6. Sorry for the poor clarification.
It is shaping up to appear that my initial thoughts that the reactors had nothing to do with the problem are true. Thank you all for your input.
RE: Shunt reactors causing CT failures?
What kind of VT do you have and how is it connected? Any time I hear "cable" and "reactor" ferroresonance is the first thing that comes to mind. Have you looked into this? Normally VTs and xfmr bushings are the failure source due to ferroresonance, but if the CT has very poor partial discharge performance (and the one you have likely does...biased opinion disclaimer) then a voltage rise due to ferroresance could have cause PD inception without extinction.
RE: Shunt reactors causing CT failures?
Are they poorly made IT's? Were both CTs and the VT made by the same manufacturer? is a 50:5 ratio appropriate to use for your application? I'm assuming you know your steady state load, but have you analyzed any transients on the system or cold load pick ups? Ferroresonance is an interesting theory, though I would tend to doubt it in the situation that you have outlined. I would hope that your ITs aren't blowing up because they are overloaded. You do have them connected to protective relays correct? Are the relay settings accurate? More information please.
First thing you should do is install a proper recording device for logging key information.
RE: Shunt reactors causing CT failures?
I normally run two cores in parallel to reduce the burden on 5A CTs and most of our new builds have 1A secondaries for this reason.
RE: Shunt reactors causing CT failures?
If you want to reduce the effects of the burden, two CTs in series will work far better. With the CTs in series each sees only half the total voltage and is therefore further from saturation.
RE: Shunt reactors causing CT failures?
RE: Shunt reactors causing CT failures?
RE: Shunt reactors causing CT failures?
I seriously doubt the CT failure has anything to do with being "over-burdened". About the only burden you can put on a CT that would cause it to fail (fail meaning insulation failure) would be an open-circuit.
RE: Shunt reactors causing CT failures?
One of my sites (fairly old one) has got a CT with a probable HV interturn fault but it hasn't failed (yet) but the output has gone high. It's only because it is a metering CT rather than protection CT that it's been noticed. It seems to be stable so not a catastrophic failure like the ones Kevin has had problems with.
If a CT open circuit is suspected then I would carry out a tightness check from the CT through the instrument/metering panel. I've removed burden resistors that had been added to a CT circuit recently as erratic power readings made me suspect a problem in this area. I'm not sure why the installers had felt it necessary to add the burden resistors.
RE: Shunt reactors causing CT failures?
maybe you have created conditions favorable for ferroresonance?....that could create voltage excursions that threaten PT installation and perhaps CT ?
Again just a thought for your consideration - I haven't studied this post carefully.
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RE: Shunt reactors causing CT failures?
"Two cores in parallel" could only mean CT secondaries, since running two conductors in parallel in a CT circuit is just asking for trouble. The solution to impedance in a CT circuit is bigger wire, not multiple wires in parallel. Never heard of such a thing and I'd never permit it on any installation I have anything to do with. Any reasonably arranged switchyard can do with 10 AWG or 8 AWG conductors for the CT circuits, but there would be no reason that a long run couldn't use 2 AWG or even larger conductors.
RE: Shunt reactors causing CT failures?
Regards
Marmite
RE: Shunt reactors causing CT failures?
Paralleling leads between CTs and instruments (meters/relays) is something I've seen a fair bit of in the US and Canada.